A brief description of the theoretical basis of this application of the said phenomenon will first be given.
Most atomic nuclei possess angular momenta together with associated magnetic dipole moments. Consider such a nucleus to be sited at the origin of a set of three mutually orthogonal co-ordinate axes, X, Y, and Z. Let the nucleus be subject to a static unidirectional magnetic field of strength, Bx, directed along the X-axis and an alternating magnetic field of peak strength, By, directed along the Y-axis. In the absence of either magnetic field, no significant component of the other field is detectable in a direction parallel to the Z-axis. In the presence of both fields however, it will be found with suitable apparatus of known art, that for every value of the field strength, Bx, there is at least one frequency, F, of the alternating field, By, at which Nuclear Magnetic Resonance occurs. The resonance is characterized by a sharp increase in the absorption of energy at the frequency, F, along the Y-axis, and by the simultaneous emission of energy at the frequency, F, along the Z-axis.
The frequency, F, is called the Larmor Precession or Resonance frequency and its ratio to the field strength, Bx, for an isolated nucleus is constant. This ratio, .gamma. = F/Bx is shown as the Magnetogyric (or Gyromagnetic) Constant for the nucleus and is sensibly independent of environmental ambient conditions of pressure, temperature, etc. Its value .gamma. for certain nuclei has been determined to a high order of precision. The constants for Hydrogen and Lithium, for example, are:
.gamma. .sub.H = 4257.76 Hz/Gs and .gamma. .sub.Li = 1654.61 Hz/Gs respectively.
The nuclei to be resonated may constitute part of the molecules of a liquid, or of a solid dissolved in a liquid, contained in a small glass enclosure (the NMR cell) which is surrounded by the coils necessary for the excitation and detection of the magnetic fields, Bx, By and Bz. In certain circumstances, the said liquid medium may be replaced by a solid medium.